1 . Field of the Invention
The present invention relates to a battery pack input/output control system. More particularly, the present invention relates to a battery pack input/output control system that limits the input/output of the battery pack, which is formed by combining a plurality of unit cells, based on a surface temperature of the battery pack.
2 . Description of the Related Art
In order to obtain a desired high-output voltage or high-output electric power, a plurality of unit cells connect in series or in parallel, or these unit cells are combined into a single assembled battery or battery pack.
The battery produces electric power through a chemical reaction. Thus, the battery generates heat during charging and discharging. An increase in temperature of the battery can adversely affect output characteristics of the battery. Therefore, the battery temperature is monitored to limit the battery input/output. In the battery pack, there are variations between the unit cells. Also, due to the construction of the battery pack, the end and the center of the battery pack exhibit different degrees of heat dissipation, causing a temperature difference between the end and the center of the battery pack. Thus, the input/output of the battery pack is limited based on a temperature that is monitored at a plurality of locations of the battery pack.
Japanese Patent Application Publication No. 11-187577 (JP-A-11-187577) discloses a charging/discharging controller of a secondary battery. The controller predetermines an upper limit of charging/discharging electric power depending on a temperature of the battery, while predetermining an upper limit of charging/discharging electric power depending on a state of charge (SOC). The battery is provided with a plurality of temperature sensors. The controller controls charge/discharge of the battery such that the charging/discharging electric power does not exceed the upper limits.
Japanese Patent Application Publication No. 2006-101674 (JP-A-2006-101674) discloses a charging/discharging controller of a secondary battery. The controller calculates an amount of internal heat generated by multiplying the square of a current value by a battery internal resistance, thus to estimate a battery temperature based on the amount of internal heat generated. The controller otherwise uses an open circuit voltage (OCV) to estimate a battery temperature that results from chemical reaction losses at the electrode-electrolyte interface. Based on higher one of the estimated battery temperature and a measured temperature, the charging/discharging electric power is limited. The limitations on the charging/discharging electric power use the relationship between an output limiting ratio of a value between 0 and 1, and the battery temperature. As the output limiting ratio is closer to 0, the charging/discharging electric power is more severely limited.
Japanese Patent Application Publication No. 11-111349 (JP-A-11-111349) discloses a battery power supply system. A battery pack is divided into a plurality of cell blocks. Based on a voltage of the whole battery pack, a voltage of each cell block, a charging/discharging current of the whole battery pack, a temperature of an individual cell, a temperature of each cell block and the like, a battery fan is controlled to maintain the battery pack at an appropriate temperature.
Japanese Patent Application Publication No. 2001-196102 (JP-A-2001-196102) discloses an assembled battery controller. The assembled battery is formed by a plurality of cell blocks. A temperature of each cell block is corrected and an internal resistance of the cell block is calculated. Based on the detected abnormal internal resistance, whether or not the temperature increases abnormally is determined.
A plurality of unit cells are combined into a battery pack. A temperature of the battery pack is monitored using a limited number of sensors that detect a surface temperature of the battery pack. Indeed, there is a difference between the surface temperature and an internal temperature of the battery, while there are variations in internal resistance between the unit cells within the battery. In addition, there are variations in contact state of the sensors with the battery surface, and there are measured temperature errors between the sensors. As described above, in the battery pack, there are variations between the individual unit cells and variations between the individual sensors. Therefore, these variations need be directly reflected in a measured temperature of the battery pack in order to determine an actual maximum internal temperature of the battery pack.
According to the foregoing related arts, in the battery pack that is formed by combining a plurality of unit cells, a plurality of sensors are used to monitor the battery temperature, and the battery temperature is estimated based on the current, while the internal resistance of the battery is calculated to monitor if the internal resistance is abnormal. Variations between the unit cells, in other words, variations in detection result from the sensors are not directly reflected in estimating the internal temperature of the battery pack based on the measured temperature of the battery pack. Thus, the internal temperature of the battery pack is not calculated correctly enough. This can possibly prevent sufficient limitations on the input/output of the battery pack. Such insufficient limitations on the input/output of the battery pack cause the battery pack to overheat, and moreover can cause smoke, if the battery pack has smoke point characteristics, for example.